In recent years, the exhaust system of some internal combustion engines is equipped with an addition valve for adding a reducing agent such as fuel into the exhaust gas. As a technique of diagnosing a failure of such an addition valve, there is a known method of diagnosing a failure of such a reducing agent addition valve in which the air-fuel ratio of the exhaust gas into which reducing agent has been added through the addition valve is detected and a failure of the addition valve is diagnosed by comparing the air-fuel ratio of the exhaust gas thus detected and a reference value (see, for example, Japanese Patent Application Laid-Open No. 2002-38928).
Japanese Patent Application Laid-Open No. 2002-161733 discloses a diagnosis technique in which the quantity of reducing agent added is feedback-controlled so that the air-fuel ratio of the exhaust gas flowing into a catalyst becomes equal to a target air-fuel ratio, and when a correction amount in the feedback control exceeds a threshold value, it is determined that the performance of the reducing agent addition valve is significantly deteriorated.
Japanese Patent Application Laid-Open No. 11-62686 discloses a diagnosis technique in which if the temperature difference between the downstream of a catalyst and the upstream of the catalyst becomes equal to or larger than a predetermined value when a reducing agent is supplied to the catalyst through a fuel injection valve of an internal combustion engine, it is determined that the fuel injection system is in an abnormal condition.
Japanese Patent Application Laid-Open No. 2005-54723 discloses a diagnosis technique in which the quantity of reducing agent added through an addition valve is estimated based on a value detected by an air-fuel ratio sensor, and an abnormality determination is made by comparing the estimated value and a criterion.
Japanese Patent Application Laid-Open No. 2003-254048 discloses a diagnosis technique in which when the time period during which the output of the air-fuel ratio sensor is equal to or smaller than a predetermined value becomes longer than a predetermined time period, it is determined that the fuel addition valve is in an abnormal condition.
Japanese Patent Application Laid-Open No. 2003-172185 discloses a system adapted to learn values of the quantity of post injection through a fuel injection valve and the quantity of reducing agent added through an addition valve with which temperature raising effects suitable for different operating states of the internal combustion engine can be achieved.